1. Field of the Invention
The present invention relates to a ultrasonic wave type obstacle sensor to sense a state of road surface in front of an automobile in advance to thereby send an input information to a suspension control device for the automobile so that a shock to the automobile body can be reduced. More particularly, the present invention relates to a ultrasonic wave obstacle sensor which avoids erroneous detection due to noises in a signal received, and which detects only an obstacle with high precision.
2. Discussion of Background
Heretofore, an obstacle sensor utilizing ultrasonic waves reflected from an obstacle has been applied to an automobile or the like. A sensor for detecting an obstacle around a car body is disclosed in, for instance, Japanese Examined Patent Publication No. 30436/1989. The sensor is adapted to generate pulsating ultrasonic waves around the car body so that judgment on the presence or absence of an obstacle is made depending on the ultrasonic waves reflected from a road surface.
A ultrasonic wave type obstacle sensor capable of sensing in advance a state of the road surface in front of the car body during cruising is disclosed in, for instance, Japanese Unexamined Patent Publication No. 31813/1987. In that publication, the sensor is used for a suspension control device for an automobile. In this case, ultrasonic waves are continuously generated obliquely and forwardly with respect to the direction of cruising of the automobile.
Generally, the distance between the sensor and an obstacle is calculated on the basis of a signal obtained by reflected waves when the obstacle is to be detected by using the ultrasonic waves. Various proposals have been made in order to measure correctly a wide range of distances which covers a short distance such as several centimeters and a long distance such as several meters.
In a proposal in the first publication, the following contrivance is made in order to avoid ultrasonic waves reflected from a normal road surface and which are recognized as an obstacle, when a state of road surface in front of or around the car body is sensed. Namely, the directively of a microphone as a signal receiving device has to be improved. Ultrasonic waves are directed in a substantially horizontal direction so that reflected waves from the normal road surface are not recognized as an obstacle. In this case, however, it is difficult to detect precisely an obstacle because the reflected waves can not be sufficiently received.
In the second publication, reflected ultrasonic waves from a road surface are positively used. However, the following disadvantages are found because the ultrasonic waves are continuously directed to the road surface in front of the car body and the reflected waves are continuously monitored.
(1) A heat value from the ultrasonic wave generating device becomes large and it is difficult to increase the ultrasonic energy.
(2) It is difficult to sense correctly an obstacle because there are interferences between the ultrasonic waves transmitted and which are received, and influenced by standing waves.
(3) It is difficult to discriminate whether the received ultrasonic waves are reflected waves from the road surface at a specified area and in front of car body, or if reflected waves from the road surface are out of the specified area (either at a remote distance or a near distance).
(4) It is difficult to discriminate factors which change the intensity of the reflected waves other than an obstacle (a projection or a concave hole) on the road surface, the factors for changing the intensity of the reflected waves being, for instance, wind, uneven distribution of temperature and so on.
Further, since the conventional obstacle sensor can not discriminate the ultrasonic waves reflected from small stones having a negligible size or noises in ultrasonic waves from another automobile near the car body when the engine of the automobile is started, it is impossible to detect correctly only the obstacle.